This invention relates to electronic tubes, particularly to an improvement of a hermetic seal structure in an electronic tube assembly wherein alumina ceramic and iron joining components are bonded together by brazing.
Generally, the sealing structure in which alumina ceramic and metal members are bonded together by brazing is widely employed in electronic tubes in order to provide a sufficient insulation between electrodes and also to enhance the bonding strength and durability.
FIG. 1 is a partial sectional view of an example of output section of a prior art magnetron in which the joint structure by brazing as mentioned above is employed. In the figure, a copper cylinder 2 which constitutes an antenna and at the same time forms a hermetic sealing section is hermetically bonded to one end of a ceramic cylinder 1 by silver brazing 3a, and to the other end of the ceramic cylinder 1 is bonded a cylinder 4 of Kovar by silver brazing 3b. To the cylinder 4 is bonded on iron cylinder 5 by silver brazing 3c.
Generally, when alumina ceramic member is bonded to a metal member, a molybdenum-based material is heated at a high temperature and sintered onto the joint surface of the alumina ceramic member to form thereon a metal surface. This metal surface is plated with nickel, for example, to provide better fluidity of a brazing material, and to prevent oxidization of the metal surface. For a joint metal member, an Fe-Ni-Co alloy (Kovar) having a thermal expansion coefficient which approximates that of the alumina ceramic is generally used. Another material for the joint metal member is an Fe-Ni alloy (Ferni) which contains Ni by approximately 42%.
Recently, the cost of Co and Ni are increasing rapidly, which in turn results in a cost up of Kovar and Ferni, threatening the production cost. In addition, raw material such as Co is in short supply.